1. Field of the Invention
The present invention relates to an antireflective film for photoelectric devices and a manufacturing method thereof, and particularly relates to an antireflective film having low reflectance with respect to the light in a wide range of wavelengths and a manufacturing method thereof.
2. Description of the Prior Art
As an example of a conventional photoelectric device, a Si solar cell 11 as shown in a sectional view of FIG. 1 will be described in the following. On a p-type Si substrate 1, an n-type Si layer 2 is formed by an impurity diffusion method or the like and a p-n junction 3 formed in the interface between the substrate 1 and the layer 2 produces a photovoltaic effect. A p-type electrode 4 is formed on the substrate 1 and an n-type electrode 5 is formed on the n-type layer 2, these electrodes serving for applying electric power to the external circuits. On the surface 2a of the n-type layer onto which light is applied, an antireflective film 6 is formed. Design and manufacturing methods of such antireflective films are described in "Semiconductors and Semimetals", Vol. 11, Solar Cells, pp. 203-207, 1975, published by ACADEMIC PRESS of New York. Generally, the antireflective film 6 is designed in the following manner. The refractive index n.sub.1 of the material of the antireflective film 6 is selected to be equal or approximate to a value represented by the relation: n.sub.1 =.sqroot.n.sub.0 .multidot.n.sub.2, where n.sub.0 is the refractive index of the air or protective cover material and n.sub.2 is the refractive index of Si. For example, in case of the air where n.sub.0 =1, the value of n.sub.1 becomes approximately 1.9 since n.sub.2 =3.6. A solar cell is practically protected and covered with resin or glass or the like (where n.sub.0 .perspectiveto.1.45) and in such case, the value of n.sub.1 becomes approximately 2.3. For the antireflective film 6 of n.sub.1 =1.9 approximately, an evaporated SiO film is generally utilized and for the antireflective film of n.sub.1 =2.3 approximately, a sputtered or thermally oxidized Ta.sub.2 O.sub.5 film or the like is utilized. The thickness d of the antireflective film 6 is set to a value obtained by the following equation so that the thickness d is adapted for the sunlight spectrum. EQU d=6000/(4.multidot.n.sub.1) .ANG.
Accordingly, the value d becomes approximately 790 .ANG. in case of SiO and approximately 650 .ANG. in case of Ta.sub.2 O.sub.5. By thus selecting the value d, the reflectance becomes nearly equal to 0 in the vicinity of the wavelength of 600 nm. However, as the wavelength deviates from such a value, the reflectance gradually increases. The Si solar cell responds to the light in a wavelength range from approximately 300 nm to 1100 nm, and in this wavelength range the average reflectance in case of using the above described antireflective film is approximately 10% (see FIG. 3). In order to further decrease the reflectance, a multilayer film including two layers or more instead of a single layer may be employed as the antireflective film. For example, if a two-layer film is employed, assuming that the refractive index of the upper layer is n.sub.1U, that the refractive index of the lower layer is n.sub.1L and that the refractive index of silicon is n.sub.2, the following relation is preferred. ##EQU1## For example, if n.sub.1U .perspectiveto.1.4 and n.sub.1L .perspectiveto.2.6, the thicknesses d.sub.1 and d.sub.2 of the upper and lower layers respectively are represented by the following equation: EQU n.sub.1U .multidot.d.sub.1 =n.sub.1L .multidot.d.sub.2 =6000/4 .ANG.
and accordingly, d.sub.1 .perspectiveto.1070 .ANG. and d.sub.2 .perspectiveto.580 .ANG. are obtained. Using the antireflective film having such values, a low reflectance of 3% on the average can be obtained in a wavelength range of 300 nm to 1100 nm. However, in reality, it is difficult to obtain materials exactly meeting such conditions. As a result, materials such as SiO.sub.2 or T.sub.i O.sub.2 having conditions approximate thereto are generally employed and consequently the reflectance becomes higher than the above described desirable value. In case of a multilayer film including two layers or more, the reflectance would increase largely if the film thickness deviates a little from the optimum values, and accordingly an extremely strict precision is needed for control of the film thickness.
As described above, in the case of a single layer antireflective film, manufacture thereof is relatively easy but the average reflectance cannot be much lowered. On the other hand, in the case of a multilayer antireflective film, the average reflectance can be considerably lowered under optimum conditions but, in order to satisfy the optimum conditions, an extremely high degree of manufacturing precision is required.